Understanding Paramagnetism and Haemocyanins

Paramagnetism and Ferromagnetism

• Paramagnetism occurs when atoms or molecules are too far apart to couple
• Spins are randomly oriented in the absence of a magnetic field
• Ferromagnetism occurs when spins orient in the same direction
• This is a long-range effect observed in materials like iron magnets
• Antiferromagnetism occurs when spins orient in opposite directions and cancel each other out
• This is a short-range effect observed in materials like oxygen

Haemocyanins

• Haemocyanins are oxygen carriers that are copper-containing proteins
• They are found in molluscs and anthropods such as crabs, snails, scorpions, and spiders
• The copper(II) binding to oxygen results in blue blood
• Horseshoe crabs have a unique haemocyanin with super antibacterial properties and a special clotting agent
• This haemocyanin is highly valuable, costing $15,000 per litre

Haemocyanin Oxygen Transfer System

• The active site of haemocyanin changes from tetrahedral to square pyramidal upon oxygen binding
• Copper is in the +1 oxidation state before oxygen binding and +2 oxidation state after oxygen binding
• Haemocyanin is diamagnetic, implying the absence of lone electrons
• The oxygen is in the peroxide form, and the spins on the copper atoms couple antiferromagnetically across the peroxide bridge, cancelling each other out

Haemocyanin and Myoglobin

• Myoglobin and haemoglobin both have a similar structure to haemocyanin
• Myoglobin contains one heme group and is a monomer
• Haemoglobin contains four heme groups and is a tetramer
• The oxygen in oxy-myoglobin is a superoxide (O2-) evidenced by its IR stretching frequency of 1107 cm-1, which is close to the superoxide IR frequency of 1140 cm-1
• This superoxide has one unpaired electron

Oxymyoglobin Complex

• Oxymyoglobin is diamagnetic because the unpaired electron in the superoxide couples with the unpaired electron from the Fe3+ in the heme group
• This results in an overall diamagnetic complex with no unpaired electrons
• The coupling between the superoxide electron and the Fe3+ electron is an example of antiferromagnetic coupling

Transition Metals in Biological Systems

• Transition metals play important roles in biological processes and medicine
• Ligands bind to transition metals in biological molecules
• The function of biological systems can be analyzed using d-orbital splitting and electron configuration

Ligand Types in Biological Molecules

• Amino acids containing oxygen in their side-chain can act as ligands to transition metals
• Other atoms with lone pairs, such as nitrogen or sulfur, can also act as ligands
• Arginine and histidine are strong-field ligands due to their high NH content

Myoglobin and Haemoglobin as Oxygen Carriers

• Myoglobin is a monomeric oxygen carrier that binds oxygen in muscle tissue
• Haemoglobin is a tetrameric oxygen carrier that transports oxygen in the blood
• Haemoglobin comprises two alpha subunits and two beta subunits, which are structurally similar to myoglobin

Iron in Oxy-Myoglobin

• Fe3+ in oxy-myoglobin has an electron configuration of [Ar]3d5, with 5 d-electrons
• The octahedral structure of oxy-myoglobin results in a low spin state for Fe3+ with only one unpaired electron
• The remaining electrons in the d-orbitals fill the lower energy t2g orbitals, and the higher energy eg orbitals remain empty